The invention relates to a method and an apparatus for the continuous, non-invasive determination of blood pressure by means of a photoplethysmographic system comprising at least one light source and at least one light detector, which are attached to a body part containing an artery.
To this day the continuous and non-invasive measuring of blood pressure presents a serious challenge to measuring technology. At the moment the so-called “Vascular Unloading Technique” is gaining more and more acceptance, which is based on a publication by Penaz (Digest of the 10th International Conference on Medical and Biological Engineering, Dresden 1973), in which light is passed through a finger and the registered flow is held constant by means of a servo-control device.
The photoplethysmographic method according to Penaz, also known as “Vascular Unloading Technique” or as “Volume Clamp Method” in some publications, has been further improved. EP 0 537 383 A1 (TNO) discloses for instance an inflatable finger cuff for non-invasive, continuous blood pressure monitoring. The inflatable cylindrical space is pneumatically connected to a fluid source. An infrared light source and a detector are placed on either side of the finger within the rigid cylinder. A valve is provided for filling the cylinder with gas. Electrical leads for the infrared light source and the detector are introduced into the cylinder. U.S. Pat. No. 4,510,940 A (Wesseling) and U.S. Pat. No. 4,539,997 A (Wesseling) show devices for the continuous, non-invasive measuring of blood pressure. A fluid-filled cuff, a light source, a light detector, a differential pressure amplifier are provided. U.S. Pat. No. 4,597,393 (Yamakoshi) also shows a variant of the Penaz principle.
WO 00/59369 A2 describes improvements of the valve control and of the pressure generating system, and various types of pressure cuffs (e.g. double cuff) for different limbs. WO 04/086963 A2 contains a description of how a double cuff can be used in such a way that in one cuff blood pressure is measured according to the Penaz principle, while the other cuff carries out optimized control of the set point (SP). WO 05/037097 A1 describes an improved control system for the vascular unloading technique, in which inner control loops present a quasi-optimized situation to the next outer control loops.
WO 2011/051822 A1 describes how signal quality of the vascular unloading technique can be improved, such that subsequently a method of pulse-shape analysis may be used for obtaining further parameters. WO 2011/051819 A1 describes an improved, exclusively digital method and device for the vascular unloading technique.
The method of Penaz has been further developed and enhanced in numerous patents and publications without however eliminating a fundamental disadvantage of the method: in order to obtain the blood pressure signal a sensor must be attached to the finger, whose contact pressure must be adapted in real time to the arterial blood pressure in the finger. Implementing this kind of fast pressure adjustment requires considerable effort and cost. All disclosures up to now use a cuff to this end, which is connected to a pump and a complex valve or valve system. The inner pressure of the cuff acting on the finger is controlled in such a way that it equals the arterial blood pressure. This is the case when the photoplethysmographic signal measured at the same time is constant.
Ideally the cuff pressure must be able to undergo changes which are as rapid as those occurring in actual arterial blood pressure, i.e. it must cope with frequencies of change in a range up to 20 Hz. This imposes costly requirements on valve or valve system, pump and cuff, which one would like to avoid. The present invention intends to substantially reduce these costs.